The present invention relates to a method of determining a deteriorated state of a catalytic converter for purifying an exhaust gas, such as a catalytic converter for use on an automobile or a hybrid vehicle.
Conventional processes of determining the deteriorated state of a catalytic converter for purifying exhaust gases produced when an air-fuel mixture is combusted, e.g., a catalytic converter disposed in the exhaust passage of an internal combustion engine, are known from Japanese patent publication No. 2,526,640 and Japanese laid-open patent publication No. 7-19033, for example.
The disclosed techniques are based on the fact that when the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine is changed from a leaner value to a richer value or from a richer value to a leaner value, the outputs from oxygen concentration sensors that are positioned respectively upstream and downstream of a catalytic converter combined with the internal combustion engine are inverted. More specifically, under certain operating conditions of the internal combustion engine, i.e., when the power output of the internal combustion engine is increased or the fuel supplied to the internal combustion engine is cut off as disclosed in Japanese patent publication No. 2,526,640 or when certain conditions are satisfied, e.g., the load and rotational speed of the internal combustion engine are in predetermined ranges as disclosed in Japanese laid-open patent publication No. 7-19033, the air-fuel ratio is positively changed from a leaner value to a richer value or from a richer value to a leaner value. At this time, the time consumed after the output of the upstream oxygen concentration sensor is inverted until the output of the downstream oxygen concentration sensor is inverted, and the period at which the output of the downstream oxygen concentration sensor is inverted are measured, and the deteriorated state of the catalytic converter is evaluated based on the measured values.
According to these techniques, when the internal combustion engine is operating under ordinary conditions, i.e., conditions without determining the deteriorated state of the catalytic converter, the air-fuel ratio is feedback-controlled depending on the inversion of the outputs from the oxygen concentration sensors in order to keep the air-fuel ratio of the internal combustion engine in the vicinity of a stoichiometric air-fuel ratio, for thereby allowing the catalytic converter to keep an appropriate purifying capability.
However, the above processes of evaluating the deteriorated state of the catalytic converter have suffered the following difficulties:
In order to determine the deteriorated state of the catalytic converter, the air-fuel ratio of the internal combustion engine needs to be positively changed to a leaner value or a richer value. Therefore, while the air-fuel ratio of the internal combustion engine is being feedback-controlled in order to allow the catalytic converter to keep an appropriate purifying capability, it is not possible to determine the deteriorated state of the catalytic converter. When the deteriorated state of the catalytic converter is determined, it is difficult to keep an appropriate purifying capability of the catalytic converter.
According to the conventional processes, the operating state of the internal combustion engine which is capable of determining the deteriorated state of the catalytic converter or the state in which exhaust gases are generated by the internal combustion engine in that operating state is limited to a certain special state. Specifically, according to the process disclosed in Japanese patent publication No. 2,526,640, the deteriorated state of the catalytic converter can be determined only if the output of the downstream O2 sensor is produced in a leaner air-fuel ratio range when the output power of the internal combustion engine is to be increased and at the time of starting to increase the output power of the internal combustion engine, and only if the output of the downstream O2 sensor is produced in a richer air-fuel ratio range when the supply of fuel to the internal combustion engine is to be cut off and at the time of cutting off the supply of fuel to the internal combustion engine.
According to the process disclosed in Japanese laid-open patent publication No. 7-19033, the deteriorated state of the catalytic converter can be determined only if the load (represented by the intake air rate, the throttle valve opening, the fuel injection quantity, and the intake air pressure) and the rotational speed of the internal combustion engine fall in a predetermined range, the intake air temperature is equal to or higher than a preset value, and the load of the internal combustion engine varies by an amount equal to or greater than a preset value. Therefore, if the internal combustion engine which generates exhaust gases to be supplied to the catalytic converter, which may be disposed in the exhaust passage of the internal combustion engine, operates in various operating states or the exhaust gases are generated in various states, then there are not many opportunities to be able to determine the deteriorated state of the catalytic converter, and the reliability of the determined deteriorated state of the catalytic converter under such conditions is low.
The applicant of the present application has proposed a system having a first exhaust gas sensor disposed upstream of a catalytic converter for generating an output representing the air-fuel ratio of an air-fuel mixture combusted by an internal combustion engine, and a second exhaust gas sensor disposed downstream of the catalytic converter for generating an output representing the concentration of a certain component of exhaust gases, e.g., the concentration of oxygen, the system being arranged to control the air-fuel ratio of the internal combustion engine according to a feedback control process to achieve an optimum purifying capability of the catalytic converter based on outputs from the sensors (see Japanese laid-open patent publication No. 9-324681, U.S. Pat. No. 5,852,930, and Japanese laid-open patent publication No. 11-93740).
The proposed system determines a target air-fuel ratio for the internal combustion engine to cause the output (the detected value of the oxygen concentration) of the second exhaust gas sensor to have a given constant value, and feedback-controls the air-fuel ratio of the internal combustion engine to converge the output (the detected value of the air-fuel ratio) of the first exhaust gas sensor to the target air-fuel ratio, for thereby achieving the optimum purifying capability of the catalytic converter.
Since the system can stably achieve the optimum purifying capability of the catalytic converter according to the above air-fuel ratio control process, it is desirable to be able to evaluate the deteriorated state of the catalytic converter while performing the air-fuel ratio control process.
It is therefore an object of the present invention to provide a method capable of appropriately evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas in various states in which an exhaust gas to be purified by the catalytic converter is generated or in various states in which an internal combustion engine that generates the exhaust gas is operated.
Another object of the present invention is to provide a method capable of appropriately evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas while maintaining a desired purifying capability of the catalytic converter which is disposed in the exhaust passage of an internal combustion engine.
To achieve the above objects, there is provided in accordance with the present invention a method of evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas produced when an air-fuel mixture is combusted, comprising the steps of supplying the exhaust gas downstream to an exhaust passage which has a first exhaust gas sensor and a second exhaust gas sensor that are disposed respectively upstream and downstream of the catalytic converter, for generating respective outputs depending on components of the exhaust gas, detecting data of the outputs of the first exhaust gas sensor and the second exhaust gas sensor when the exhaust gas is supplied to the exhaust passage, identifying the value of at least one parameter to be set of a model that is constructed as representing a behavior of an object exhaust system which ranges from the first exhaust gas sensor to the second exhaust gas sensor and includes the catalytic converter the exhaust passage, based on the detected data of the outputs of the first exhaust gas sensor and the second exhaust gas sensor, and determining a deteriorated state of the catalytic converter based on data of the identified value of the parameter.
Studies made by the inventors indicate that a model expressing the behavior of the object exhaust system including the catalytic converter and ranging from the first exhaust gas sensor to the second exhaust gas sensor is constructed, and when the value of the parameter to be set, i.e., the parameter to be set to a certain value in defining the behavior of the model, is sequentially identified based on the data of the outputs of the exhaust gas sensors that are acquired while the exhaust gas is being supplied to the exhaust passage, the identified value of the parameter has a certain characteristic correlation to the deteriorated state of the catalytic converter (e.g., as the deterioration of the catalytic converter progresses, the magnitude of the identified value of the parameter increases or decreases). The value of the parameter can basically be identified without fail based on the data of the outputs of the exhaust gas sensors in a situation where the outputs of the exhaust gas sensors vary to a certain extent. Therefore, it is possible to identify the value of the parameter when the exhaust gas to be supplied to the catalytic converter is generated in various states or the internal combustion engine operates in various operating states.
Preferably, the first exhaust gas sensor comprises a sensor for producing an output representing the air-fuel ratio of the air-fuel mixture from which the exhaust gas entering the catalytic converter is produced, and the second exhaust gas sensor comprises a sensor for producing an output representing the content of a particular component of the exhaust gas that has passed through the catalytic converter.
When the value of the parameter of the model of the object exhaust system which employs the above sensors as the first and second exhaust gas sensors is identified based on the data of the outputs of the first and second exhaust gas sensors at the time the exhaust gas is supplied to the exhaust passage, a relatively distinct correlation tends to appear easily between the identified value and the deteriorated state of the catalytic converter. Therefore, it is easy to determine the deteriorated state of the catalytic converter based on the data of the identified value of the parameter.
Since it is possible to determine the deteriorated state of the catalytic converter when the exhaust gas to be supplied to the catalytic converter is generated in various states or the internal combustion engine operates in various operating states, the catalytic converter is preferably disposed in the exhaust passage of the internal combustion engine which combusts the air-fuel mixture therein.
If the first exhaust gas sensor comprises a sensor for producing an output representing the air-fuel ratio of the air-fuel mixture, and the second exhaust gas sensor comprises a sensor for producing an output representing the content of a particular component of the exhaust gas that has passed through the catalytic converter, then the method preferably further comprises the step of controlling the air-fuel ratio of the internal combustion engine in order to converge the output of the second exhaust gas sensor to a predetermined target value when the exhaust gas is supplied to the exhaust passage upon operation of the internal combustion engine, wherein the value of the parameter is identified and the deteriorated state of the catalytic converter is determined concurrent with the step of controlling the air-fuel ratio of the internal combustion engine.
By controlling the air-fuel ratio of the internal combustion engine, or more specifically the air-fuel ratio of the air-fuel mixture combusted in the internal combustion engine, to converge the output of the second exhaust gas sensor which represents the content of the particular component of the exhaust gas having pass through the catalytic converter, it is possible to achieve a desired purifying capability of the catalytic converter for purifying the exhaust gas emitted from the internal combustion engine. When the value of the parameter is identified and the deteriorated state of the catalytic converter is determined concurrent with the step of controlling the air-fuel ratio of the internal combustion engine, the deteriorated state of the catalytic converter can be determined while maintaining the desired purifying capability of the catalytic converter during operation of the internal combustion engine.
If an oxygen concentration sensor (O2 sensor) is used as the second exhaust gas sensor, then an optimum purifying capability of the catalytic converter is achieved by controlling the air-fuel ratio of the internal combustion engine to keep the output of the sensor at a given constant level.
The step of controlling the air-fuel ratio of the internal combustion engine preferably comprises the steps of calculating a target air-fuel ratio of the internal combustion engine in order to converge the output of the second exhaust gas sensor to the target value, and controlling the air-fuel ratio of the internal combustion engine according to a feedback control process in order to converge the air-fuel ratio represented by the output of the first exhaust gas sensor to the target air-fuel ratio.
By thus controlling the air-fuel ratio of the internal combustion engine, the air-fuel ratio detected by the first exhaust gas sensor can stably be controlled at an air-fuel ratio suitable to achieve the desired purifying capability of the catalytic converter, i.e., the target air-fuel ratio. Since the air-fuel ratio of the internal combustion engine is stably controlled, the behavior of the data of the outputs of the first and second exhaust gas sensors which are used to identify the value of the parameter is made smooth. As a result, the effect of disturbances other than the deteriorated state of the catalytic converter on the identified value of the parameter is reduced. Consequently, the deteriorated state of the catalytic converter can appropriately be evaluated based on the deterioration evaluating parameter which represents the degree of variation of time-series data of the identified value of the parameter.
While the target air-fuel ratio can be calculated using a PID controller, it is preferably calculated by a sliding mode controller.
Specifically, the sliding mode controller is advantageous in that it is more resistant to disturbances than the PID controller. The target air-fuel ratio calculated by the sliding mode controller makes stable the process of controlling the air-fuel ratio of the internal combustion engine. As a result, the desired purifying capability of the catalytic converter can be achieved more reliably. At the same time, the identified value of the parameter is made reliable. Thus, the deteriorated state of the catalytic converter can be determined more adequately based on the data of the identified value.
In controlling the air-fuel ratio of the internal combustion engine concurrent with determining the deteriorated state of the catalytic converter, the target air-fuel ratio is preferably calculated by an algorithm determined in advance using the identified data of the parameter.
Specifically, since the identified value of the parameter reflects the actual behavioral characteristics of the object exhaust system, when the target air-fuel ratio for converging the output of the second exhaust gas sensor to the target value is calculated using the identified value, the accuracy of the target air-fuel ratio is increased. As a consequence, the desired purifying capability of the catalytic converter can be achieved more reliably. At the same time, the stability of the identified value of the parameter is increased, and the deteriorated state of the catalytic converter can be determined more adequately based on the data of the identified value.
While the air-fuel ratio of the internal combustion engine can be feedback-controlled by a PID controller, it is preferably controlled by a recursive-type controller.
When the air-fuel ratio of the internal combustion engine is feedback-controlled by a recursive-type controller, or specifically an adaptive controller, it is possible to feedback-control the air-fuel ratio detected by the first exhaust gas sensor more accurately at the target air-fuel ratio while suppressing the effect of characteristic changes of the internal combustion engine than if a PID controller is used. The desired purifying capability of the catalytic converter can be achieved more reliably, and the reliability of the identified value of the parameter is increased, so that the deteriorated state of the catalytic converter can be determined more adequately based on the deterioration evaluating parameter.
The recursive-type controller determines a new manipulated variable according to a given recursive formula including time-series data in the past prior to the present of a manipulated variable for the air-fuel ratio of the internal combustion engine, or more specifically a manipulated variable for the fuel supply quantity of the internal combustion engine, for example, in order to converge the air-fuel ratio represented by the output of the first exhaust gas sensor to the target air-fuel ratio, and controls the air-fuel ratio of the internal combustion engine with the manipulated variable.
The model comprises a model expressing the object exhaust system as a discrete-time system for generating the output of the second exhaust gas sensor from the output of the first exhaust gas sensor via a response delay element and/or a dead time element, and includes, as the parameter, at least one of a coefficient relative to the output of the first exhaust gas sensor and a coefficient relative to the output of the second exhaust gas sensor.
By thus constructing the model of the object exhaust system and the values of the coefficients used in the model as the parameter are identified based on the data of the outputs of the first and second exhaust gas sensors, the identified value of the parameter (coefficients) of the model accurately reflects the actual behavioral characteristics of the catalytic converter included in the exhaust system. As a result, the correlation between the identified value and the deteriorated state of the catalytic converter is increased. Therefore, the deteriorated state of the catalytic converter can be determined adequately based on the identified value of the parameter (coefficients). By modeling the object exhaust system as a discrete time system, the value of the parameter (coefficients) can be identified on a real-time basis.
With the object exhaust system being thus modeled, the step of identifying the value of the parameter comprises the steps of sequentially identifying the value of the parameter according to an algorithm for sequentially updating and identifying the value of the parameter in order to minimize an error between the output of the second exhaust gas sensor in the model and an actual output of the second exhaust gas sensor, and filtering the output of the second exhaust gas sensor in the model and the actual output of the second exhaust gas sensor with the same frequency passing characteristics in calculating the error.
It is thus possible to identify the value of the parameter (coefficients) in a manner to cause the frequency characteristics of the actual object exhaust system including the catalytic converter and the model, or more specifically the frequency characteristics of changes of the output of the second exhaust gas sensor (corresponding to the output of the model) with respect to changes of the output of the first exhaust gas sensor (corresponding to the input of the model), to match each other. Thus, the identified value of the parameter is highly reliable, and the deteriorated rated state of the catalytic converter can be determined more adequately based on the identified value.
The step of identifying the value of the parameter preferably comprises the step of identifying the value of the parameter depending on a particular behavior of the object exhaust system.
Depending on the behavior of the object exhaust system, the identified value of the parameter may lack reliability. By identifying the value of the parameter in a certain behavior of the object exhaust system, i.e., a behavior in which air-fuel ratio of the air-fuel mixture recognized by the oxygen concentration in the exhaust gas changes from a leaner value to a richer value, the identified value of the parameter is made highly reliable as reflecting the behavioral characteristics of the object exhaust system. Thus, the reliability of the identified value of the parameter is increased, and the deteriorated state of the catalytic converter can be determined highly reliably based on the identified value.
The step of identifying the value of the parameter preferably comprises the step of recognizing the particular behavior of the object exhaust system based on the value of a function that is determined by a predetermined number of time-series data prior to the present of the output of the second exhaust gas sensor.
The step of identifying the value of the parameter preferably comprises the step of limiting the identified value of the parameter.
The above process makes it possible to prevent the deteriorated state of the catalytic converter from being determined based on an unduly identified value of the parameter and a.less reliable identified value of the parameter, with the result that the reliability of the determined deteriorated state of the catalytic converter is increased. If the air-fuel ratio of the internal combustion engine is controlled using the identified value of the parameter, then the stability of the process of controlling the air-fuel ratio of the internal combustion engine is increased.
The step of identifying the value of the parameter preferably comprises the step of calculating the identified value of the parameter based on the difference between an actual output of the first exhaust gas sensor and a predetermined reference value and the difference between an actual output of the second exhaust gas sensor and a predetermined reference value, which differences are used as the data of the outputs of the first and second exhaust gas sensors.
In calculating the identified value of the parameter, the difference between the actual output of the first exhaust gas sensor and the predetermined reference value and the difference between the actual output of the second exhaust gas sensor and the predetermined reference value are used as the data of the outputs of the first and second exhaust gas sensors. In this manner, an algorithm for calculating the identified value can be constructed relatively easily, and the accuracy of the identified value is increased.
As described above, for controlling the air-fuel ratio of the internal combustion engine in order to converge the output of the first exhaust gas sensor to a given target value, the reference value relative to the first exhaust gas sensor is preferably set to the above target value.
The above techniques according to the present invention described above with respect to the construction of the model and the identifying step may be combined with the technology of types of the first and second exhaust gas sensors and the various technologies relative to the air-fuel ratio control of the internal combustion engine.
The deteriorated state of the catalytic converter may be determined based on the identified value of the parameter by comparing the identified value with a suitable decision value. Preferably, the step of determining the deteriorated state of the catalytic converter comprises the step of using data generated by effecting a predetermined filtering process on the data of the identified value of the parameter as data for determining the deteriorated state of the catalytic converter.
The identified value of the parameter itself may fluctuate even if the deteriorated state of the catalytic converter remains the same due to a disturbance. By using the data produced by effecting a predetermined filtering process on the data of the identified value of the parameter as data for determining the deteriorated state of the catalytic converter, the deteriorated state of the catalytic converter can be determined highly reliably.
The filtering process may be a process for determining an average or a weighted average of the identified value of the parameter as the determining data. However, it is particularly preferable that the filtering process comprise a process of determining a central value of the least square of the data of the identified value of the parameter. When the above filtering process is carried out, the determining data is more distinctly correlated to the deteriorated state of the catalytic converter, and hence the deteriorated state of the catalytic converter can be determined more adequately based on the determining data.
The method which uses the above determining data should further comprise the step of classifying and recognizing the flow rate of the exhaust gas supplied to the exhaust passage when the value of the parameter is identified, according to a plurality of regions, wherein the step of determining the deteriorated state of the catalytic converter comprises the steps of effecting the filtering process on the data of the identified value of the parameter in each of the regions to generate the data for determining the deteriorated state of the catalytic converter, and determining the deteriorated state of the catalytic converter using the data, which is generated in at least one of the regions, for determining the deteriorated state of the catalytic converter.
Specifically, the identified value of the parameter is somewhat affected by the flow rate of the exhaust gas passing through the catalytic converter 3, e.g., the magnitude of the identified value of the parameter is somewhat increased or decreased as the flow rate of the exhaust gas increases. Therefore, the flow rate of the exhaust gas is classified into a plurality of ranges, e.g., a large flow rate range and a small flow rate range, and determining data are generated respectively for the flow rate ranges that are recognized when the identified value of the parameter is determined. The deteriorated state of the catalytic converter is determined using the determining data that is generated with respect to at least one of the flow rate ranges. In this manner, the reliability of the determined result is increased, and the deteriorated state can be determined irrespective of the flow rate of the exhaust gas.
More specifically, the step of determining the deteriorated state of the catalytic converter comprises the step of determining the deteriorated state of the catalytic converter based on the magnitude of the data of the identified value of the parameter and/or the characteristics of changes of the value of the data, the changes attending on the progress of the deterioration of the catalytic converter
Alternatively, if the deterioration determining data is generated by effecting a filtering process on the data of the identified value of the parameter, then the step of determining the deteriorated state of the catalytic converter comprises the step of determining the deteriorated state of the catalytic converter based on the magnitude of the data for determining the deteriorated state of the catalytic converter and/or the characteristics of changes of the value of the data for determining the deteriorated state of the catalytic converter, the changes attending on the progress of the deterioration of the catalytic converter.
Specifically, the data of the identified value of the parameter or the deterioration determining data generated from the data of the identified value of the parameter has the magnitude of its value or the pattern of its changes characteristically correlated to the deteriorated state of the catalytic converter, and the correlation is basically determined depending on the type of the catalytic converter or the type of the parameter that is identified. Therefore, the deteriorated state of the catalytic converter can appropriately be determined by taking into account the magnitude of the value of the above data and the characteristics of its changes.
More specifically, if a model is constructed which expresses the object exhaust system as a discrete-time system for generating the output of the second exhaust gas sensor from the output of the first exhaust gas sensor via a response delay element and/or a dead time element, and if the parameter includes the coefficient relative to the output of the first exhaust gas sensor, then the data of the identified value of the coefficient or the magnitude of the value of data generated by effecting a predetermined filtering process on the data of the identified value exhibits an increasing tendency or a decreasing tendency as the deterioration of the catalytic converter progresses until the deterioration of the catalytic converter progresses to a certain extent. Thereafter, the tendencies are reversed, and hence the data exhibits a decreasing or increasing tendency as the deterioration of the catalytic converter progresses. Thus, when the deterioration of the catalytic converter progresses to a certain extent, the value of the above data becomes an extremal value (maximum or minimum value). In this case, therefore, the step of determining the deteriorated state of the catalytic converter comprises the step of determining the deteriorated state of the catalytic converter based on the data of the identified value of the coefficient relative to the output of the first exhaust gas sensor or the magnitude of the value of data generated by effecting a predetermined filtering process on the data of the identified value, and information about an extremal value (maximum or minimum value) occurring in the value of the data as the deterioration of the catalytic converter progresses, e.g., information indicative of whether the extremal value is produced or not.
If the above model of the object exhaust system is constructed and if the parameter includes the coefficient relative to the output of the second exhaust gas sensor, then the data of the identified value of the coefficient or the magnitude of the value of data generated by effecting the above filtering process on the data of the identified value exhibits a monotonous increasing tendency or a monotonous decreasing tendency as the deterioration of the catalytic converter progresses. In this case, therefore, the step of determining the deteriorated state of the catalytic converter comprises the step of determining the deteriorated state of the catalytic converter based on the data of the identified value of the coefficient relative to the output of the first exhaust gas sensor or the magnitude of the value of data generated by effecting a predetermined filtering process on the data of the identified value.
If the model has a plurality of parameters to be identified, then the step of determining the deteriorated state of the catalytic converter comprises the step of determining the deteriorated state of the catalytic converter based on data of identified values of the parameters.
With this arrangement, the reliability of the determined result is higher than if the deteriorated state of the catalytic converter is determined based on the data of the identified value of a single parameter.
More specifically, the step of determining the deteriorated state of the catalytic converter comprises the steps of temporarily determining deteriorated states of the catalytic converter based on the respective data of the identified values of the parameters, and thereafter determining the deteriorated state of the catalytic converter based on a combination of the temporarily determined deteriorated states. Consequently, the deteriorated state of the catalytic converter can easily be determined highly reliably based on the data of the respective identified values of the plural parameters.
The step of determining the deteriorated state of the catalytic converter preferably comprises the step of classifying and determining the deteriorated state of the catalytic converter in a plurality of degrees of deterioration. The degree of deterioration of the catalytic converter can thus be recognized stepwise.
The step of determining the deteriorated state of the catalytic converter preferably comprises the steps of determining whether the identified value of the parameter is appropriate for determining the deteriorated state of the catalytic converter or not based on at least one of the data of the output of the first exhaust gas sensor and the data of the output of the second exhaust gas sensor, and determining the deteriorated state of the catalytic converter based on the identified value of the parameter which is determined as being appropriate.
Alternatively, if the catalytic converter is disposed in the exhaust passage of the internal combustion engine, then the step of determining the deteriorated state of the catalytic converter preferably comprises the steps of determining whether the identified value of the parameter is appropriate for determining the deteriorated state of the catalytic converter or not based on at least one of the data of the output of the first exhaust gas sensor, the data of the output of the second exhaust gas sensor, and an intake quantity of the internal combustion engine, and determining the deteriorated state of the catalytic converter based on the identified value of the parameter which is determined as being appropriate.
If the internal combustion engine is mounted as propulsive source on a motor vehicle, then the step of determining the deteriorated state of the catalytic converter preferably comprises the steps of determining whether the identified value of the parameter is appropriate for determining the deteriorated state of the catalytic converter or not based on at least one of the data of the output of the first exhaust gas sensor, the data of the output of the second exhaust gas sensor, an intake quantity of the internal combustion engine, and a vehicle speed of the motor vehicle, and determining the deteriorated state of the catalytic converter based on the identified value of the parameter which is determined as being appropriate.
In a situation where the output of the first exhaust gas sensor and the output of the second exhaust gas sensor are steadily substantially constant, and variations of these outputs are extremely small, it is difficult to determine the data of the parameter highly accurately from those output data, and the identified value tends to suffer an error. A situation where the outputs of the exhaust gas sensors are substantially constant is liable to take place if the internal combustion engine is operating such that the intake quantity of the internal combustion engine with the object exhaust system disposed in the exhaust passage, or more accurately the intake quantity per combustion cycle, or if the vehicle speed of the motor vehicle on which the internal combustion engine is mounted as propulsive source is steadily substantially constant.
According to the present invention, it is determined whether the identified value of the parameter is appropriate for determining the deteriorated state of the catalytic converter based on at least one (preferably a plurality of) of the data of the output of the first exhaust gas sensor, the data of the output of the second exhaust sensor, the intake quantity of the internal combustion engine, and the vehicle speed of the motor vehicle, and the deteriorated state of the catalytic converter is determined based on the identified value of the parameter that is determined as appropriate. Therefore, the reliability of the identified value of the parameter can be increased, and hence the reliability of the determined deteriorated state of the catalytic converter can be increased.
The step of determining whether the identified value of the parameter is appropriate for determining the deteriorated state of the catalytic converter or not may comprise the step of determining whether the identified value of the parameter is appropriate for determining the deteriorated state of the catalytic converter or not, based on whether at least one of the output of the first exhaust sensor and the output of the second exhaust sensor is maintained at a substantially constant level when the data of the outputs of the first and second exhaust gas sensors used to determine the identified value of the parameter are detected. In a situation where the output of the first exhaust gas sensor, etc. is kept substantially constant at the time the data of the outputs of the first and second exhaust gas sensors used to determine the identified value are acquired, the identified value of the parameter may be determined as being not appropriate. Otherwise, preferably in a situation a plurality of the output of the first exhaust gas sensor, etc. vary to a certain extent, the identified value of the parameter may be determined as being appropriate.
The above techniques according to the present invention described above with respect to the deterioration determining step may be combined with the technology of types of the first and second exhaust gas sensors, the technologies relative to the air-fuel ratio control of the internal combustion engine, and the techniques relative to the construction of the model or the processing of the identifying step.
The method according to the present invention described above preferably further comprises the step of indicating the determined deteriorated state-of the catalytic converter for thereby presenting a prompt to replace the catalytic converter.